The structure of a vacuum electron tube is known, as illustrated by FIG. 1. An electron-emitting cathode Cath and an anode A are arranged in a vacuum chamber E. A potential difference V0, typically between 10 KV and 500 KV, is applied between the anode A and the cathode Cath to generate an electrical field E0 inside the chamber, allowing the extraction of the electrons from the cathode and the acceleration thereof, to produce an “electron gun”. The electrons are attracted to the anode under the influence of the electrical field E0. The electrical field generated by the anode has 3 functions:
extractions of the electrons from the cathode (for the cold cathodes),
to give a trajectory to the electrons for them to be used in the tube. For example, in a TWT, that makes it possible to inject the electron beam into the interaction impeller,
to give energy to the electrons through the voltage gradient for the needs of the tube. For example, in an X-ray tube, the energy of the electrons controls the X-ray emission spectrum.
A TWT is a tube in which an electron beam transits in a metal impeller. An RF wave is guided in this impeller in order to interact with the electron beam. This interaction results in a transfer of energy between the electron beam and the RF wave which is amplified. A TWT is therefore a high-power amplifier, that is found for example in telecommunications satellites.
In an X-ray tube, according to one embodiment, the electrons are braked by impact on the anode, and these decelerated electrons emit an electromagnetic wave. If the initial energy of the electrons is strong enough (at least 1 keV), the associated radiation is in the X range. According to another embodiment, the energetic electrons interact with the core electrons of the atoms of the target (anode). The electron reorganization induced is accompanied by the emission of a photon of characteristic energy.
Thus, the electrons emitted by the cathode are accelerated by the external field E0 either towards a target/anode (typically made of tungsten) for an X-ray tube, or to an interaction impeller for a TWT.
In order to produce a (quasi-)continuous emission of electrons, two technologies are employed: (i) cold cathodes and (ii) thermoionic cathodes.
Cold cathodes are based on an electron emission by field emission: an intense electrical field (a few V/nm) applied to a material allows a curvature of the energy barrier that is sufficient to allow the electrons to transit to the vacuum by tunnel effect. Obtaining such intense fields macroscopically is impossible.
Cathodes with vertical tips use the field emission combined with the tip effect. For this, a geometry that is very widely used and developed in the literature consists in producing vertical tips P (with a strong aspect ratio) on a substrate as illustrated by FIG. 2. By tip effect, the field at the tip of the emitter can be of the order sought. This field is generated by the electrostatic disturbance represented by the tip in a uniform field. In this configuration, a uniform external field E0 is applied. It is the variation of this field which makes it possible to control the field level at the tip of the emitters and therefore the corresponding emitted current level.
The first gated cathodes, called Spindt tips, were developed in the 1970s and are illustrated in FIG. 3. Their principle is based on the use of a conductive tip 20 surrounded by a control gate 25. Typically, the apex is on the plane of the gate. It is the potential difference between the tips and the gate which makes it possible to modulate the electrical field level at the apex of the tips (and therefore the current emitted). These structures are known for their very high sensitivity to the tip/gate alignment and for the problems of electrical insulation between the 2 elements.
More recently, tip emitters have been produced from carbon nanotubes or CNTs, arranged vertically, at right angles to the substrate.
A gated cathode with carbon nanotubes CNT is also described for example in the patent application No PCT/EP2015/080990 and illustrated in FIG. 4. A gate G is arranged around each VACNT (for “Vertically Aligned CNT”).
The field emission results from the electrical field on the surface of a typically metallic material. Now, this field is directly linked to the gradient of the electrical potential field applied.
In a conventional cathode (no gate), the potential field results from the combination of the influences of the external field and from the potential of the nanotube alone. Now, these two are linked.
In a cathode of “gated” type, the potential field at the level of the nanotubes results from the combination of the influences of the external electrical field, from the potential of the nanotube (as previously) but also from the potential induced by the gate which is independent of the other two. Thus, it is possible to modify the electron emission level by acting with this new electrode introduced into the system.
Generally, the field amplification factor associated with each emitter is strongly linked to its height and to the radius of curvature of its tip. Dispersions in these two parameters induce amplification factor dispersions. Now, the tunnel effect is an exponential law involving this amplification factor: thus, by considering a cohort of emitters, only a fraction (which can be relatively low, of the order of one percent or less) really participates in the electron emission. For a target total current, this requires the actual emitters to be able to emit relatively high currents (compared to an emission which would be uniform and distributed uniformly over all the emitters).
The production of these emitters in tip form is done:
either directly on the substrate, by etching (e.g.: silicon tips), by direct growth (example: CNT). These two methods have to allow a preferential orientation of the tips at right angles to the substrate;
or by mounting: synthesis of a nanomaterial (in nanotube/nanowire form) then mounting on a substrate. A step of orientation at right angles to the substrate is also necessary.
With a production directly on substrate, significant radius/height dispersions are known in the literature. In addition, in the specific case of the CNTs grown on substrate, the orientation at right angles to the substrate is controlled but the quality of the material is notably lower than that of the CNT material obtained by CVD growth. One means of reducing the height dispersion is to perform a polishing on encapsulated material: the drawback lies in the fact that the polished material is defective, which reduces the associated emission performance levels.
In the case of materials grown then mounted on substrate, obtaining an orientation at right angles to the substrate is complex (not localized, actual height uncontrolled, etc.).
Cathodes that have a planar geometry (no object orientation at right angles to the substrate) based on nanowire, known from the literature, are still based on the tip effect. However, in order to mitigate the orientation not at right angles to the substrate, a counter-electrode to the electrode bearing the emitter is incorporated in the substrate. A first example is illustrated in FIG. 5: an emitter of Pp tip type, of ZnO nanowire type, is parallel to the substrate. One of its ends is connected to an electrode (cathode Cath) and a counter-electrode (anode A) makes it possible to generate the equivalent of the homogeneous field E0 in the case of the vertical structures. The emission still appears at the apex of the tip. The electron beam is propagated from the emitter to the anode, it is possible but difficult to deflect the beam to use it elsewhere (notably to inject it into a conventional electron tube). Another example operating according to the same principle, comprising a gate G and a tip Pp of doped polysilicon, is illustrated in FIG. 6.
In the case of a vacuum tube, the aim is to use the electron beam “far” from the cathode. In the case of a planar structure, the anode is in direct proximity to the emissive element (in order to limit the voltages to be applied) which means that the beam travels a very short distance before being intercepted by the anode. It cannot therefore be used further away in the vacuum tube.
The thermoionic cathodes use the thermoionic effect to emit electrons. This effect consists in emitting electrons through heating. For that, the two electrodes arranged at the ends of a filament are biased. The application of a potential difference between the two ends generates a current in the filament, which heats up through Joule's effect. When it reaches a certain temperature (typically 1000 degrees Celsius) electrons are emitted. In effect, simply the fact of heating allows some electrons to have a thermal energy greater than the metal-vacuum barrier: thus, they are spontaneously extracted to the vacuum.
There are cathodes in pad form (of the order of one millimetre) with an electric filament placed underneath to ensure the heating of the material, which will then emit electrons.
The thermoionic cathodes make it possible to supply high currents over long periods in relatively medium vacuums (up to 10−6 mbar for example). However, their emission is difficult to switch rapidly (on the scale of a fraction of a GHz for example), the size of the source is fixed and their temperature limits the compactness of the tubes in which they are incorporated.
One aim of the present invention is to mitigate the drawbacks mentioned above by proposing a vacuum electron tube having a planar cathode based on nanotubes or nanowires that makes it possible to overcome a certain number of limitations linked to the use of vertical emitting tips, while using the tunnel effect or the thermoionic effect or a combination of the two.